2р(х-7)-р(2х)=2рх-14р-2р-2рх=-16р
<span> 7x+2=3x+1</span>
a7=a1+6d=21
S7=(a1+a7)/2*7=205
система:
a1+6d=21
(a1+a7)/2*7=205
a1=205/7
d=-58/21